Clearly, the depth of focus (DOF) and spot size of a laser machining process is determined by the focal length of its optical module, the wavelength of the laser beam, input beam size, and so on. Since the DOF of an optical device for a laser machining process is usually proportional to the spot size of the laser machining process, i.e. when a high precision laser machining is performed using a small spot size, the DOF of the optical module for the laser machining is usually very small. Nevertheless, in a laser machining process performed under the restriction of a very small DOF, any careless operation for moving a mobile component in the laser machining platform or even a slight deflect to the flatness of the film to be machined can cause the machining to excess the range of the DOF, causing the machining quality to drop significantly. For instance, in a laser machining process using ultraviolet laser working in conjunction with an objective lens module, the resulting spot size is about 5 um and the DOF is ranged between 15 um˜20 nm.
There are already many studies for solving the aforesaid problem. One of which is an apparatus for laser machining process, disclosed in U.S. Pat. No. 6,706,998, entitled “Simulated Laser Spot Enlargement”, in which the apparatus employs an fast steering mirror in the beam path to continuously move the laser beam in a high speed prescribed pattern about a nominal target position to spatially separate the focused laser spots generated at a high laser repetition rate and thereby create geometric features having dimensions greater than those of the focused laser spot, and thereby, permits a series of laser pulses at a given repetition rate to appear as a series of larger diameter pulses at a lower pulse rate without the beam quality problems associated with working out of focus, i.e. the laser spot size can be enlarged effectively for increase the area that can be treated within a specific period of time. Moreover, another such study is an apparatus for laser machining process, disclosed in U.S. Pat. No. 7,498,238, entitled “Chip and Method for Dicing Wafer into Chips”, in which a laser head composed of a laser beam source and a condenser lens module is mounted on a vertical machining platform for enabling the same to be displaced up and down in a vertical direction of a wafer, and thereby the DOF range of the laser machining apparatus can be increased for facilitating the dicing of the wafer. In addition, there is an adjustable laser beam delivery system and method for forming the same disclosed in WO/2007/108589, in which the width and length of a focused laser spot can be adjusted and varied along with the varying of a distance associated with a convex lens in a lateral direction and in a vertical direction. Furthermore, a dual-focus micro-machining method is disclosed in Journal of Modern Optics, November 2005, pp. 2603-2611(9), by B. Tan and K. Venkatakrishnan, in which an optical configuration of plate beam splitter, convex mirror and focusing lens is provided for generating dual-focus from a single laser incident beam, and thereby the generated two foci have nearly equivalent spot size and both fall on the optical axis of the focusing optics, but at different focal lengths. The dual-focus optics allows for variations of the laser power of each focal point and the distance between the two focal points. The advantages of dual-focus ablation were demonstrated with a nanosecond UV laser dicing silicon substrates.